The transfer of a slurried solid from a first liquid phase into a second liquid phase, said first and second liquid phases being immiscible, and the solid being insoluble in both of the liquid phases, is a well known technique called "flushing".
In the present specification, the term "pigment" designates the pigment per se plus an optional content of resins added during or after the chemical synthesis of the pigment. Especially in the case of yellow and red pigments, resins are added. The character and nature of the resins are explained in detail herinafter.
It is known to flush pigments from a press-cake in which the pigment constitutes about 20-50% into a non-aqueous phase. The flushing may be performed by mixing the two liquid phases (containing the solid), e.g. by means of a dough mixer, a high-speed stirrer, a static mixer, etc. The resulting slurry of pigments in the non-aqueous phase normally comprises 20-40% of pigment. The advantages of flushed pigments are mainly based on the fact that the fine structure of the pigments is retained in the flushed form.
The initial steps for manufacturing either dry or flushed pigment colours are identical. Chemical intermediates are reacted together to form the pigment, which is in most cases isolated as an aqueous suspension (the slurry) and filtered through a filter press or other filtering device such as a centrifuge where the pigment solids are separated and a press-cake or pulp is formed. Whereas the original slurry normally contains from 3 to 6% by weight of the solids, the press-cake contains from 20 to 50% by weight of the pigment solids. The press-cake is washed with water in order to remove soluble impurities (the cake is then sometimes blown with air in the filter) and the press-cake is removed from the filter for future processing.
The separation process carried out by various methods of filtration often involves undesired compaction of the pigment particles.
The initial aqueous suspension of pigment can be formed in various ways such as: (a) by precipitating a dye (a lake pigment) with the salt of a metal (barium chloride, calcium chloride, lead nitrate, etc.), (b) by forming an insoluble pigment from a diazo coupling reaction, (c) by the absorption of a dye on an inorganic hydrate (such as aluminium hydrate), or (d) by aqueous dilution of a dispersed or solvated pigment (for example phthalocyanine blue).
Proceeding from the press-cake or pulp stage, dry and flushed colour processing go their separate ways. If dry colour is desired, the press-cake is dried and pulverized. These later stages may give rise to environmental problems caused by the dusting of the dry pigment.
In the know process, in order to flush the pigment the press-cake is transferred to the flushing equipment. The non-aqueous medium is added in a predetermined amount, and the mixing process is performed. When the non-aqueous medium replaces most of the water, the phases separate, and the aqueous phase is removed and discarded. The flushing process often has to be repeated in order to form a "dense" flushed mass, i.e. a mass with a sufficiently high concentration of pigment in the non-aqueous medium.
A low water content in the flushed mass can be achieved by a vacuum treatment.
GB PS No. 1,342,746 (to ICI) describes dispersions of inorganic pigments of lakes or toners in organic liquids which contain dissolved therein one or more polyesters or a salt thereof derived from a hydroxycarboxylic acid of the formula: EQU HO--X--COOH
wherein X is a divalent saturated or unsaturated aliphatic radical containing at least 8 carbon atoms and in which there are at least 4 carbon atoms between the hydroxy and carboxylic acid groups, or from a mixture of such a hydroxycarboxylic acid and a carboxylic acid which is free from hydroxy groups.
GB PS No. 1,373,660 (to ICI) describes similar dispersing agents, i.e. of the formula YCOZR wherein Z is a divalent bridging group connected to the carbonyl group via an oxygen or nitrogen atom, R is a primary, secondary or tertiary amino group or a salt thereof with an acid or a quaternary ammonium salt group, and Y is the residue of a polyester chain which together with the group --CO-- is derived from a hydroxycarboxylic acid of the formula EQU HO--X--COOH
wherein X is a divalent saturated or unsaturated aliphatic radical containing at least 8 carbon atoms and in which there are at least 4 carbon atoms between the hydroxy and carboxylic acid groups, or from a mixture of such a hydroxycarboxylic acid and a carboxylic acid which is free from hydroxy groups.
In the cited documents there is no mentioning of flushing carried out on the aqueous phase per se in which the pigment is synthesized. When flushing has been performed hitherto it has been carried out on pastes, press-cakes or "pulps", i.e. on mixtures of pigments and aqueous phases in which the amount of pigment is much larger than is the case in the present invention.